This invention relates generally to transport refrigeration systems and, more particularly, to supplying electrical power to all the power demand loads of the transport refrigeration unit while reducing engine fuel consumption.
Refrigerated trucks and hailers are commonly used to transport perishable cargo, such as, for example, produce, meat, poultry, fish, dairy products, cut flowers, and other fresh or frozen perishable products. A transport refrigeration system is mounted to the truck or to the trailer in operative association with a cargo space defined within the truck or trailer for maintaining a controlled temperature environment within the cargo space.
Conventionally, transport refrigeration systems used in connection with refrigerated trucks and refrigerated trailers include a transport refrigeration unit having a refrigerant compressor, a condenser with one or more associated condenser fans, an expansion device, and an evaporator with one or more associated evaporator fans, which are connected via appropriate refrigerant lines in a closed refrigerant flow circuit. Air or an air/gas mixture is drawn from the interior volume of the cargo space by means of the evaporator fan(s) associated with the evaporator, passed through the airside of the evaporator in heat exchange relationship with refrigerant whereby the refrigerant absorbs heat from the air, thereby cooling the air. The cooled air is then supplied back to the cargo space.
On commercially available transport refrigeration systems used in connection with refrigerated trucks and refrigerated trailers, the compressor, and typically other components of the transport refrigeration unit, must be powered during transit by a prime mover. In the case of refrigerated trailers, the prime mover typically comprises a diesel engine carried on and considered part of the transport refrigeration system. In mechanically driven transport refrigeration systems the compressor is driven by the diesel engine, either through a direct mechanical coupling or a belt drive, and other components, such as the condenser and evaporator fans are belt driven.
An all electric transport refrigeration system for refrigerated trailer application is also commercially available through Carrier Corporation headquartered in Farmington, Conn., USA. In the all electric transport refrigeration system, a prime mover, most commonly a diesel engine, carried on and considered part of the transport refrigeration system, drives an AC synchronous generator that generates AC power. The generated AC power is used to power an electric compressor motor for driving the refrigerant compressor of the transport refrigeration unit and also powering electric AC fan motors for driving the condenser and evaporator motors and electric heaters associated with the evaporator. For example, U.S. Pat. No. 6,223,546 discloses an all electric transport refrigeration system.
In conventional practice, a transport refrigeration unit installed on a refrigerated truck or trailer operates in one of a temperature pulldown mode, a temperature maintenance mode, or a standstill mode. In the temperature pulldown mode, the refrigerant compressor, the condenser fan(s) and the evaporator fan(s) are operating with the refrigerant compressor generally operating at full capacity to lower the temperature within the cargo space as rapidly as possible to a desired set point temperature appropriate for the particular cargo stowed in the cargo space. In the temperature maintenance mode, the refrigerant compressor, the condenser fan(s) and the evaporator fan(s) are still operating, but the refrigerant compressor is operating at a significantly lower capacity so as to maintain the temperature in the cargo space within a specified range of the desired set point temperature and avoid over cooling. In the temperature maintenance mode, heaters associated with the evaporator may also be activated as necessary to warm the air passed through the evaporators by the evaporator fan(s) to prevent over cooling. In the standstill mode, the refrigerant compressor and the condenser and evaporator fans are off.
Diesel engines used as prime movers on transport refrigeration systems generally have two operating speeds, that is a high RPM speed, such as 2200 RPM, and a low RPM speed, such as 1400 RPM. In operation, the diesel engine is operated at high speed during temperature pulldown and at low speed during the temperature maintenance mode. During standstill, the diesel engine is typically idling at low speed. The diesel engine is generally designed to meet the power needs of the transport refrigeration system during operation at maximum capacity, such as during the temperature pulldown mode, with efficient fuel consumption. Therefore, during the temperature maintenance mode and standstill mode, the diesel engine is operating at lower efficiency and with increased fuel consumption.